Apparatus and method for producing purified hydrogen gas by a pressure swing adsorption processes

ABSTRACT

The invention relates to a method and an apparatus for producing purified hydrogen gas by a pressure swing adsorption process. Further the invention relates to detecting an operating life of adsorbents in a adsorption tower. The method and the apparatus have a gas supply unit for adding an inert gas to an unpurified hydrogen gas and a detector for measuring an inert gas in a purified hydrogen gas discharged from the adsorption tower.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for producingpurified hydrogen gas by a pressure swing adsorption process. Morespecifically, the present invention relates to detecting an operatinglife of adsorbents.

2. Description of Related Art

Pressure swing adsorption (PSA) processes are used for purifyinghydrogen gas. In PSA processes, hydrogen gas included with impuritygases is fed to an adsorption tower filled with an adsorbent and thehydrogen gas passes through the adsorbent at a high pressure, whileimpurity gases, for example, N₂, Ar, CH₄ and chlrosilanes(dichlorosilane (abbreviated “DCS”), trichlorosilane, tetrachlorosilane(abbreviated “STC”), etc), are adsorbed by the adsorbent at highpressure. After a predetermined time has passed, a feed of unpurifiedhydrogen gas is turned off. The adsorption tower is depressurized bytypically one or more steps, while a purified hydrogen gas is dischargedfrom the adsorption tower. After that, the adsorbent is regenerated bydepressurizing and purging with pure hydrogen gas and by heating up theadsorbent. A purified hydrogen gas is produced by repeating these steps,as described in U.S. Pat. No. 6,261,343 B1. Industrially, the PSAprocess includes multiple adsorption towers, with each tower connectedto on-off valves and constantly produces a purified hydrogen gas, as inU.S. Publication No. 2007/0204748 A1.

Over time, the adsorption capacity of the adsorbents is reduced byrepeating the PSA process. Finally, before the adsorbents areregenerated, impurities pass through the adsorbents without beingadsorbed. This is know as the “breach” phenomenon.

PSA processes are used in many industry fields. Especially, thepolycrystalline silicon manufacturing industry commonly uses PSAprocesses. Polycrystalline silicon is produced by feedingtrichlorosilane (SiHCl₃, abbreviated “TCS”) gas and purified hydrogengas onto a surface of silicon seed rods at a high temperature in areactor. This method is known as the Siemens method and is shown byfollowing main reaction formulas:SiHCl₃+H₂→Si+3HCl (Si bulk surface)  (1)SiHCl₃+HCl

SiCl₄+H₂ (Gas phase)  (2)

In this method, exhausted gas from the reactor includes unreactedhydrogen gas. The unreacted hydrogen gas is purified by the PSA processand the purified hydrogen gas is reused as raw material gas forproducing polycrystalline silicon.

In the polycrystalline silicon producing process, impurities arestrictly controlled. It is important to detect an operating life of theadsorbents before a breach is caused. As a method for detecting abreach, Japan Publication Application Nos. S63-144110, H07-277720, and2001-58118 propose methods in which a content of hydrogen chloride inthe purified hydrogen is measured. But, lately, a method that moreaccurately detects impurity breach and an operating life of theadsorbents is needed.

One object of this present invention is to provide an apparatus and amethod for detecting the operating life of adsorbents. Another object ofthis present invention is to provide an apparatus and a method forpurifying a hydrogen gas for a polycrystalline silicon producingprocesses.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus for producing purifiedhydrogen gas by a pressure swing adsorption process, the apparatushaving: (A) a gas supply unit for adding an inert gas to an unpurifiedhydrogen gas; (B) an adsorption tower filled with an adsorbent forpurifying unpurified hydrogen gas fed into the adsorption tower; and (C)a detector for measuring an inert gas in a purified hydrogen gasdischarged from the adsorption tower.

The present invention relates to an apparatus, for producing apolycrystalline silicon, which purifies hydrogen gas discharged from areaction of depositing polycrystalline silicon and feeds the hydrogengas back to the reaction, the apparatus having: (A) a reactor fordepositing polycrystalline silicon on silicon seed rods by reactingtrichlorosilane with hydrogen gas; (B) a condenser for separating anunpurified hydrogen gas discharged from the reactor; (C) a gas supplyunit for adding an inert gas to the unpurified hydrogen gas; (D) anadsorption tower filled with an adsorbent for purifying unpurifiedhydrogen gas fed into the adsorption tower; and (E) a detector formeasuring an inert gas in a purified hydrogen gas discharged from theadsorption tower.

The present invention further relates to a method for producing purifiedhydrogen gas by a pressure swing adsorption process, the method havingthe steps of: (A) adding an inert gas to an unpurified hydrogen gas; (B)feeding the unpurified hydrogen gas into an adsorption tower andpurifying by pressure swing adsorption; and (C) detecting the inert gasin the purified hydrogen gas discharged from the adsorption tower.

The present invention also relates to a method for producingpolycrystalline silicon, which purifies hydrogen gas discharged from areaction of depositing polycrystalline silicon and feeds it back to thereaction, the method having the steps of: (A) depositing polycrystallinesilicon on silicon seed rods by reacting trichlorosilane with hydrogengas in a reactor; (B) separating an unpurified hydrogen gas bycondensing a discharged gas from the reactor; (C) adding an inert gas tothe unpurified hydrogen gas; (D) feeding the unpurified hydrogen gasinto an adsorption tower and purifying by pressure swing adsorption; and(E) detecting the inert gas in the purified hydrogen gas discharged fromthe adsorption tower.

The invention detects an operating life of adsorbents by using an inertor noble gas, like Ar gas, for example. Inert Ar gas is a very weakadsorption element. If the adsorbents are close to their adsorptioncapacity, Ar gas passes through the adsorbents and can be detected by anAr gas detector. From a change in a pattern of content of inert gas,like Ar gas for example, it is possible to accurately decide when theadsorbents should be replaced with new ones without getting anycontamination in purified H₂.

Especially, the invention is also suitable for the polycrystallinesilicon producing process. A polycrystalline silicon producing processis required to strictly prevent contaminating impurities. In thepolycrystalline silicon process, polycrystalline silicon is deposited ona silicon seed rod by reacting chlorosilane with hydrogen gas in areactor, and unreacted hydrogen gas is separated and fed back to thereactor. Unreacted hydrogen gas needs to be purified before being fedback to the reactor. The present invention provides an apparatus and amethod for detecting an operating life of adsorbents by using an inertgas, like Ar gas, for example. Ar gas does not react with chlorosilaneor hydrogen gas and does not affect the depositing of polycrystallinesilicon on a silicon seed rod. Further, Ar gas does not impact on theoperation of polycrystalline silicon production if the volume of Ar inpurified H₂ is negligible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the invention;

FIG. 2 is an illustration of the polycrystalline silicon producingprocess of the invention; and

FIG. 3 is a chart of Ar content detected by Gas Chromatography-ThermalConductivity Detector (GC-TCD) in a four adsorbent tower system withthree operational towers and one tower off-line.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic representation of an adsorption tower in a PSAapparatus according to the present invention. The PSA apparatuscomprises an inert gas supplying unit 3, an adsorption tower 1 filledwith an adsorbent or adsorbents and an inert gas, like Ar gas, detector5. The inert gas supply unit 3 consists of a compressed inert gascylinder 3 a, filled with an inert gas like, Ar gas for example, and aflow regulating valve 3 b. The inert or Ar gas supply unit 3 isconnected to an H₂ feeding pipe or line 2.

The adsorption tower 1 has a plurality of adsorbent beds, typically 4,10 or 12 beds. Zeolite, activated carbon and carbon molecular sieves,etc., are used as adsorbents. The adsorption tower 1 is connected to theH₂ feeding line 2 and a regeneration discharging line 7 at the bottomthereof, and is connected to a purified H₂ discharging line 4 and aregeneration gas feeding line 6 at the top thereof. The adsorbent bedsare commonly divided into three zones for adsorbing different weights ofimpurities. A lower zone commonly adsorbs heavy weight impurities likeSTC or TCS; an intermediate zone adsorbs intermediate weight impuritieslike SiH₃Cl, SiH₄, PH₃, or AsH₃; and an upper zone adsorbs light weightimpurities like N₂ or Ar.

The inert gas detector, in this case the Ar detector 5, for example, isprovided in the purified H₂ discharging line 4. A GC-TCD, density meteror thermal conductivity gas analyzer, etc., is used as the Ar detector5.

The PSA apparatus of present invention works in the following way. Whilehydrogen gas is being purified, feeding H₂ line valve 2 a and purifiedH₂ discharging line valve 4 a are open and the regenerated gas feedingvalve 6 a and the regenerated gas discharging line valve 7 a are closed,so that unpurified hydrogen gas is continuously fed, for example, atabout 2.41 MPa*m³/s under high pressure of about 0.69 MPa, by absolutepressure, to the adsorption tower 1 and passes through adsorbent beds(not shown). Impurity compounds are adsorbed on the adsorbent beds, andthe purified hydrogen gas is discharged from the adsorption tower 1through purified H₂ discharging line 4.

While hydrogen gas is being purified, the flow regulating valve 3 b isopen and Ar gas, for example, is adding to the unpurified hydrogen gasfrom compressed Ar gas cylinder 3 a. Ar gas is supplied, for example, atabout 11.50 Pa*m³/s (200 ppm(vol)), more preferable at about 8.63Pa*m³/s (150 ppm(vol)). In early stages of purifying hydrogen gas, apart of Ar gas is adsorbed in the adsorption beds. But because Ar gas isa weak adsorption element, Ar gas can pass through the adsorption bedsand is detected by the Ar detector 5. Ar gas content in adsorptiontowers versus time is shown in FIG. 3.

In the end of the purifying step, the adsorbents become close to theiradsorption capacity. After a predetermined time has passed, the feedingH₂ line valve 2 a is closed and Ar gas stops feeding to the adsorptiontower 1. Next, the purified H₂ discharging line valve 4 a is closed andthe regenerated gas discharging line valve 7 a is open. Then, a pressureinside of the adsorption tower 1 is depressurized, for example, to 0.19MPa by absolute pressure, from regenerated gas discharging line 7. Sothat the pressure inside of the adsorption tower 1 is depressurized, forexample, from 0.69 MPa to 0.19 MPa from regenerated gas discharging line7.

In a regenerating step, a regenerated gas feeding valve 6 a is open anda purge gas is fed into the adsorption tower 1. In this invention,hydrogen gas is used as purge gas. After that, the regenerated gasdischarging line valve 7 a is closed and a pressure inside of theadsorption tower 1 is increased, for example, up to 0.69 MPa by absolutepressure.

These adsorption, depressurization and regeneration steps are repeated,and purified hydrogen gas is continuously made in the PSA process steps.If the adsorbents are close to an end of their operating life by therepetition of these PSA process steps, the Ar content chart of Ar gas toadsorption tower pressure changes to different curves. Especially, inthe early step of adsorption, if Ar content is increased, it is a signalthat the adsorbents are close to the capacity of adsorption. Theadsorbents can be replaced before the purified hydrogen gas iscontaminated by impurities.

FIG. 2 is an illustration of polycrystalline silicon producing processadapting the PSA process in the purified hydrogen producing step. Thispolycrystalline silicon producing process comprises a fluidized bedchlorinator 11, a distillation series 12 comprising a plurality ofdistillation towers, a vaporizer 16, a polycrystalline silicon reactor10, a condenser 14, and a distillation tower 13.

TCS is produced in the fluidized bed chlorinator 11 by reactingmetallurgical grade silicon powder (abbreviated “Me-Si”) of about 98%purity fed via line 40 with a hydrogen chloride gas (HCl) fed via line41. The TCS is purified in the distillation series 12 and a vaporfraction of the distillation, as the purified TCS, is fed to thevaporizer 16 via feed line 30. In the vaporizer 16, the purified TCS isvaporized and mixed with H₂ and the mixed gas is fed to thepolycrystalline silicon reactor 10 as raw material gases. Thepolycrystalline silicon reactor 10 has a plurality of polycrystallinesilicon seed rods (not shown). The silicon seed rods are heated and thepurified TCS and H₂ are fed into the reactor 10. Polycrystalline siliconis deposited on the silicon seed rods. Exhaust gases from the reactioninclude unreacted TCS, unreacted H₂, HCl, STC (SiCl₄), DCS (SiH₂Cl₂) andother chlorosilanes. The exhaust gases are fed to the condenser 14 andchlorosilane groups, such as TCS, STC, DCS and other chlorosilanes arecondensed, so that they are separated from the gases including H₂ andHCl in the condenser 14. The condensed chlorosilane groups are fed tothe distillation tower 13 and TCS is separated and is purified in thedistillation tower 13. The purified TCS in the distillation tower 13 isfed to the vaporizer 16 via feed line 31 and is reused as raw materialgas.

The gases separated in the condenser 14, including H₂ and HCl, are fedto a HCl adsorbing tower 15. In the HCl adsorbing tower 15, most of HClis adsorbed and other gases are fed to adsorption towers 17 a, b, c andd (abbreviated “adsorption towers 17”). An inert gas, like Ar gas,supply unit 23 is located between the HCl adsorption tower 15 and theadsorption towers 17. However, the Ar supply unit 23 is able to belocated upstream of the HCl adsorbing tower 15, as well. The Ar supplyunit 23 consist of a compressed Ar cylinder 23 a and a flow regulatingvalve 23 b. The gases discharged from the HCl adsorbing tower 15comprise, for example, approximately 83.59% H₂, 0.16% HCl, 12.95% TCS,0.07% STC, 3.23% DCS, by weight. The Ar supply unit 23 supplies Ar gasto the gases not less than about 0.05%, and not more than about 0.50% byweight, for example. More preferably, Ar gas is supplied to the gasesbetween about 0.25% and about 0.35% by weight, for example.

The adsorption towers 17 are filled with an adsorbent, like activatedcarbon (not shown), for example. In this embodiment, four adsorptiontowers 17 a, b, c and d are arranged in a row. Each adsorption tower hasvalves 19, 20, 21 and 22, respectively indicated a, b, c and d for eachrespective adsorption tower 17 a, b, c and d, in the inlet and theoutlet of the tower thereof. Each adsorption tower is operated on adifferent adsorption step from each other by controlling the valves 19,20, 21 and 22. For example, when the adsorption tower 17 a is on theadsorption step, the gases from the HCl adsorption tower 15 pass throughthe adsorption tower 17 a by opening the valves 20 a and 21 a andclosing the valves 19 a and 22 a. While the gases pass through theadsorption tower 17 a, impurities, such as HCl, TCS, STC, DCS and otherchlorosilanes are trapped by the activated carbon at around the bottombed, and hydrogen is purified in the adsorption tower 17 a. Theadsorption tower 17 b is depressurized by opening the valve 19 b andclosing the valves 22 b, 20 b, and 21 b, then purging is started withpurified H₂ in tower 17 b by opening valve 22 b for regeneration, whilethe adsorption tower 17 a is on the adsorption step. Regeneration gasfrom tower 17 b through valve 19 b is recycled after purified by anotheradsorption tower, and returned back (not shown) to the inlet of the HCladsorption tower 15. At the same time, the adsorption tower 17 c ispressurized by opening the valve 22 c and closing the valves 19 c, 21 cand 20 c, and the adsorption tower 17 d is off-line in a wait step(stand-by) by opening valve 21 d and closing valves 19 d, 20 d and 22 d.An example of such a four adsorption tower system is show in FIG. 3.Valves 19, 20, 21 and 22 can be controlled, as explained above, toproduce the gas flow, cycle timing, and Ar content as shown in FIG. 3.

The purified hydrogen, which passed through the adsorption towers 17, isfed to the vaporizer 16 via a feed line 32 and is reused as rawmaterial. Some of the purified hydrogen is returned back to theadsorption towers 17 from feed line 32 via a return line 33 for theregeneration of the adsorption towers 17. Purity of the purifiedhydrogen is 99.7% and Ar content is 0.3% by weight, for example.

Inert gas detectors, in this case Ar gas detectors 18, for detecting Argas discharged from the adsorption towers 17, are provided downstream ofthe adsorption towers 17. In this embodiment, GC-TCD is used as the Ardetector 18.

A working example of the above system is shown in FIG. 3. FIG. 3 shows agraph of tower pressure (MPaG) of three adsorption towers (abbreviated“TWR”) A, C and D and Ar gas weight content (wt %) versus time (minutes)in a four consecutive tower operation, with tower B in off-line,stand-by mode. Each tower operates in different cycles, adsorption,depressurization and regeneration, at different times. Chronologicallyin FIG. 3, Ar gas content varies in each tower over time. For example,in TWR-C, a smaller Ar adsorption than in other towers is observed atthe beginning of the adsorption cycle by a decrease to 0.2 wt %,followed by a larger Ar gas breach than in other towers shown by thepeak in Ar wt % at above 0.3 wt %. The maximum weight percent of Ar inthe next tower, TWR-D, is less than TWR-C and more than the last tower,TWR-A. At maximum Ar breach, TWR-C and TWR-D are in the adsorptioncycles, while TWR-A is in a H₂ purge cycle. At the next highest wt % ofAr observed in TWR-D, both TWR-D and TWR-A are in adsorption cycles,while TWR-C is in the H₂ purge cycle. Finally, at the lowest wt % amountof Ar, TWR-C and TWR-A are in the adsorption cycle while TWR-D is in theH₂ purge cycle.

The data of FIG. 3 is shown below in Table 1:

TABLE 1 Time TWR-D TWR-C TWR-A Ar Minutes MPaG MPaG MPaG wt % 0 0.090.58 0.59 0.27 30 0.59 0.58 0.08 0.2 32 0.59 0.58 0.08 0.25 90 0.59 0.580.08 0.26 120 0.59 0.58 0.15 0.27 122 0.59 0.58 0.15 0.27 124 0.59 0.580.15 0.32 140 0.59 0.58 0.12 0.29 208 0.59 0.58 0.09 0.28 210 0.59 0.580.09 0.28 212 0.59 0.58 0.09 0.28 272 0.59 0.58 0.10 0.27 303 0.59 0.080.59 0.17 305 0.59 0.08 0.59 0.24 363 0.59 0.08 0.59 0.24 393 0.59 0.150.59 0.25 395 0.59 0.15 0.59 0.25 397 0.59 0.15 0.59 0.29 413 0.59 0.120.59 0.28 481 0.59 0.09 0.59 0.27 485 0.59 0.09 0.59 0.27 487 0.59 0.090.59 0.27 542 0.59 0.10 0.59 0.26 572 0.08 0.59 0.59 0.17 574 0.08 0.590.59 0.24 632 0.08 0.59 0.59 0.24 662 0.15 0.59 0.59 0.25 664 0.15 0.590.59 0.25 666 0.15 0.59 0.59 0.28 682 0.12 0.59 0.59 0.27 750 0.09 0.590.59 0.26 754 0.09 0.59 0.59 0.26 756 0.09 0.59 0.59 0.26 814 0.10 0.590.59 0.26 844 0.59 0.59 0.08 0.2

The adsorption cycle is repeated in the order of TWR-C, TWR-D and TWR-A.Smaller amounts of Ar gas adsorption and larger amounts of Ar gas breachare indications of adsorbent carbon deterioration. In the graph, Arcontent changes as shown by the two inflections and two plateaus pertower in the Ar curve in FIG. 3. The first plateau is stable due to therecycled regeneration gas which contains higher amounts Ar gas. Thesecond plateau is stable due to the saturation of breached Ar gas. Atclose to the adsorption capacity of adsorbent, the Ar gas content ofTWR-C shows a higher amount of Ar gas breach. FIG. 3 shows a large Argas breach at 0.32 wt %. This is a signal to replace the adsorbent tonew adsorbent.

At the minimum, at least two carbon towers are necessary in order tooperate pressure swing adsorption systems.

The embodiments and examples are described for illustrative, but notlimitative purposes. It is to be understood that changes and/ormodifications can be made by those skilled in the art without for thisdeparting from the related scope of protection, as defined by theenclosed claims.

1. A method for producing purified hydrogen gas by a pressure swingadsorption processes, comprising: adding an inert gas to an unpurifiedhydrogen gas; feeding the unpurified hydrogen gas into an adsorptiontower and purifying by pressure swing adsorption; and detecting theinert gas in the purified hydrogen gas discharged from the adsorptiontower.
 2. The method according to claim 1, wherein the inert gas is Argas.
 3. The method according to claim 2, wherein Ar gas is detected inthe range of about 0.15 to 0.35 wt %.
 4. The method according to claim2, wherein Ar gas is detected under an adsorption tower pressure rangeof about 0.59 to 0.76 MPaG.
 5. A method for producing polycrystallinesilicon, which purifies hydrogen gas discharged from a reaction ofdepositing polycrystalline silicon and feeds the hydrogen gas back tothe reaction, comprising: depositing polycrystalline silicon on siliconseed rods by reacting trichlorosilane with hydrogen gas in a reactor;separating an unpurified hydrogen gas by condensing a discharged gasfrom the reactor; adding an inert gas to the unpurified hydrogen gas;feeding the unpurified hydrogen gas into an adsorption tower andpurifying by pressure swing adsorption; and detecting the inert gas inthe purified hydrogen gas discharged from the adsorption tower.
 6. Themethod of claim 5, further comprising returning some of the dischargedpurified hydrogen gas back to the adsorption tower for regenerating theadsorption tower.
 7. The method according to claim 5, wherein the inertgas is Ar gas.
 8. The method according to claim 7, wherein Ar gas isdetected in the range of about 0.15 to 0.35 wt %.
 9. The methodaccording to claim 7, wherein Ar gas is detected under an adsorptiontower pressure range of about 0.59 to 0.76 MPaG.